Various automatic coffee brewing control systems are available in the industry. Some early systems incorporated spring-loaded mechanical timers to control brewing cycles to obtain various strengths of the coffee product (see, e.g., U.S. Pat. No. 2,448,149). Other coffee brewing systems have used liquid level sensors to control various functions. For example, the level of liquid in a coffee brewer is described in U.S. Pat. No. 3,060,835 to stop the flow of heated water into the brewer, after which a "ready" lamp would be energized after a time delay to allow residual water to pass through the brewer. In other devices (e.g., U.S. Pat. Nos. 4,682,537 and 4,825,758), the level of liquid in the water reservoir has been used to energize the heating element during the time the water is present within the reservoir.
In some commercial automatic coffee brewing systems, the amount of time that the brewed coffee remains "fresh" is very important. Rather than using brewing time to obtain varying strengths of the coffee product (as was done in the prior art), the critical time increment in the latter commercial applications begins when the coffee product has completed brewing and continues for the maximum amount of time for which the brewed coffee is considered fresh. The concept of "fresh" has varying definitions, however, the maximum time period for maintaining already-brewed coffee fresh is generally considered to be in a range of about thirty (30) to forty-five (45) minutes.
In applications where it is important to know how long a fluid has been in a container (e.g., perishable fluids such as coffee, chemical mixtures, carbonated beverages, adhesives, dairy products, etc.), conventional methods would dictate that an operator of the system start a timer at the time the container is filled, and later note the elapsed time when it is determined that the fluid is no longer fresh. As the container is moved to other locations, in order to dispense the fluid, the container and/or timer may not be visible or easily tracked in use, and persons using or dispensing the fluids may not have confidence in the freshness of the fluid. As used herein, the term "perishable" shall connote any fluid or mixture of fluids having a critical residence time or limited useable life of any sort in a container. While the example of coffee and "freshness" is set forth herein to illustrate the principles involved, it should be understood that other fluids may have various other critical characteristics which vary with time and which must be monitored. For example, adhesives may have optimum time following mixture during which they remain pliable, spreadable, or otherwise useable (e.g., epoxy resins and hardeners).
Heretofore, there has not been available a container which can independently and reliably detect when it is full of fluid (with its own self-contained means), and which can also independently monitor the condition of the fluid within the container for freshness or other characteristics during a predetermined period of time after being placed into the container. To be most useful, such container should also have the capability of discontinuing the "fresh" signal once the container is emptied of the fluid.